The present invention relates to a method for forming an insulating film on a semiconductor substrate surface. More particularly, the present invention relates to a method for forming a silicon nitride film which exhibits an excellent stability in electrical properties and a high degree of density, on a surface of a semiconductor substrate useful for the production of integrated circuits.
In the production of semiconductor devices, it is known that a portion of the semiconductor surface, in which a PN junction is exposed to an ambient atmosphere, is coated with an insulating film in order to prevent changes in the electrical properties of the semiconductor device with the lapse of time and to enhance the reliability of the semiconductor device. In the production of MISFET (metal insulator semiconductor field effect transistor) or MIS type integrated circuits, it is also known that a gate insulating film is formed in the semiconductor device, so as to introduce an inversion layer channel into the device. Generally speaking, in the conventional MISFET and MIS type integrated circuits, the semiconductor substrate, the insulating film and the metal gate electrode respectively consist essentially of silicon (Si), silicon oxide (SiO.sub.2) and aluminum (Al). However, the above-mentioned structure of the conventional metal insulator semiconductor device exhibits a disadvantage in that an application of an electric field of about 10.sup.6 V/cm onto the insulating film at a temperature of about 200.degree. C. results in a significant fluctuation of the gate threshold voltage. This disadvantageous phenomenon may be due to the fact that the drift effect of impurity ions contained in the SiO.sub.2 insulating film or the structural defects of the SiO.sub.2 insulating film per se cause the creation of a capture level of carriers, and the creation of the cature level of carriers results in a significant fluctuation in the space charge distribution in the surface layer of the semiconductor device. It is clear that the fluctuation in the space charge distribution is influenced significantly by the structural defects in the interface between the Si substrate and the insulating film (SiO.sub.2). Also, structural defects are created to a significant extent when the insulating film (SiO.sub.2) is formed by thermally oxidizing the surface layer of the Si substrate. This thermal oxidation process tends to permit the impurity ions, such as alkali metal ions, to be contaminated in the oxidized silicon film. In order to eliminate the above-mentioned defects from the conventional insulating film, an attempt was made to provide an insulating film by way of a chemical vapor deposition or sputtering. However, all of the previous attempts failed to reduce the structural defects in the interface between the Si substrate and the SiO.sub.2 layer to a level lower than that of the thermal oxidation.
Furthermore, it is known that when the surface of the Si substrate is thermally oxidized in an extremely clean atmosphere, the resultant SiO.sub.2 film exhibits low structural defects at a level, in terms of surface charge density, of 10.sup.11 /cm.sup.2 or less. However, the interface between the Si substrate and the SiO.sub.2 film still exhibits structural defects due to excessive silicon ions. Accordingly, it has been strongly desired to eliminate the above-mentioned defects from the interface between the Si substrate and the SiO.sub.2 film.
On the other hand, recently, the increase in density of the integrated circuits is accompanied with a tendency to decrease the thickness of the gate insulating film in the semiconductor device. When a thinner insulating film is used, it is important that the resultant semiconductor device exhibits an excellent stability in electrical properties.
As a method for decreasing the variation in the electrical properties of the semiconductor device, it was attempted to provide a gate insulating film consisting of a silicon oxide film formed on a silicon substrate and a phospho-silicate glass (PSG) film formed on the silicon oxide film. However, in the case where the PSG film has a high concentration of phosphorus, an application of a high electric field onto the gate insulating film results in polarization of the gate insulating film. This polarization causes the potential of the semiconductor surface to undesirably fluctuate. Also, it was found that the reduction in the thickness of the silicon oxide film causes the operation of the resultant N channel enhancement type field effect transistor to be more difficult due to the diffusion of donor impurities from the polysilicon gate film into the silicon substrate.
In another method, it was tried to utilize a silicon nitride film or aluminum oxide film as a gate insulating film. However, since the above-mentioned films are provided by means of chemical vapor deposition, structural defects are locally created in the interface between the silicon substrate and the insulating film. These structural defects cause the electrical properties of the semiconductor device to be unstable.
Moreover, it was attempted to utilize a silicon nitride film which was produced by bringing a nitrogen or an ammonia gas atmosphere into contact with a surface of a silicon substrate heated to an elevated temperature so as to perform a nitridation reaction and to convert the surface portion of the silicon substrate into a silicon nitride. However, the silicon nitride film produced by the above-mentioned nitridation method sometimes exhibits structural defects in the interface between the silicon substrate and the silicon nitride film. Also, it is sometimes difficult to produce a silicon nitride film having an even structure and composition. The above-mentioned structural defects or the structural uneveness cause the electrical properties of the semiconductor device to be unstable.